Partial inactivation of glutamine synthetase (GS) by a mixed function oxidase model system composed of ascorbate, O2 and Fe(III) leads to the formation of hybrid GS molecules (dodecamers) composed of both inactive and active subunits. Subunit interaction in these hybrid molecules are weaker than in the native enzyme, as is indicated by the kinetics of subunit dissociation in the presence of 4 M urea. Heterologous subunit interactions in these hybrid molecules do not affect the affinity of active subunits for glutamate. Incubation of partially adenylylated glutamine synthetase preparations with the ascorbate system in the absence of substrates leads to preferential oxidation (inactivation) of unadenylylated subunits, whereas incubation in the presence of ATP and glutamate leads to preferential inactivation of adenylylated subunits.